Conventional pneumatic tires may contain a built-in sealant layer based upon a depolymerized butyl rubber layer. For example, see U.S. Pat. Nos. 4,895,610, 4,228,839, 4,171,237, 4,140,167, 8,156,979, and 8,293,049 and U.S. Patent Publication Nos. 2003/0230376, 2004/0159386, 2005/0205186, and 2008/0115872. Additional patent publications which propose various tire constructions which may involve built-in or built-on sealants for tires, such as for example, U.S. Pat. Nos. 1,239,291, 2,877,819, 3,048,509, 3,563,294, 4,206,796, 4,286,643, 4,359,078, 4,895,610, 4,919,183 and 4,966,213. As a conventional tire rotates, centrifugal force may promote a small degree of flow of the built-in sealant layer located in the shoulder or sidewall regions of the tire toward the center, or crown region, of the tire, thereby reducing the puncture sealing capability of the built-in sealant layer in the shoulder region of the tire. Also, after a tire has stopped rotating the warm, low viscosity sealant flows under gravity to the lowest point in the tire, thereby inducing an imbalance into the tire which is detrimental to tire ride and comfort.
A conventional tire may have a unitary built-in sealant layer divided into zones, namely, an annular central zone positioned in the crown region of the tire and annular lateral zones, wherein the built-in sealant of the lateral zones may be a higher storage modulus (G′) material than the sealant of the central zone to present a greater dimensional stability and resistance to flow of the sealant in the lateral zones promoted by centrifugal force resulting by rotation of the tire. The lateral zones of the sealant layer may be individually positioned on each axial side of the central zone. The lateral zones of the sealant layer may be individually positioned axially outward from the center of the sealant layer wherein a central zone extends over the entire axial width of the sealant layer. The storage modulus (G′), at a 5 percent dynamic strain at 100° C. and 1 hertz of the sealant composition of the lateral sealant zones to be least 15, alternately, at least 20.0 kPa higher (greater) than the sealant composition of the central sealant zone.
The zones of the built-in sealant layer of pneumatic tires may be derived from a depolymerization of a butyl rubber-based sealant precursor composition, typically containing a rubber reinforcing carbon black filler to render the sealant black in color or containing precipitated silica with only a minimal amount of carbon black, if any, or exclusive of carbon black, together with a colorant to color the sealant layer a color other than black. For such a zoned sealant layer, amount of organoperoxide efficiency for the in situ depolymerization of the butyl rubber may be instructive.
By controlling the amount of the organoperoxide, free radical promoted butyl rubber depolymerization activity, or rate, (referred to herein as organoperoxide activity) may be varied and the degree (extent) of depolymerization of the butyl rubber varied, depending upon the selection of organoperoxide for the individual sealant zones to thereby result in the sealant compositions of the individual sealant zones having different storage modulus (G′) values. Accordingly, an organoperoxide for a sealant precursor of the central zone may have a greater activity (a more active organoperoxide) than the organoperoxide of the lateral zones.
For example, where the organoperoxide for the lateral zones is comprised of dicumyl peroxide for the in situ formation of the built-in sealant of the lateral zones, a more active organoperoxide (such as for example n-butyl-4,4-di(tert-butyl-peroxy) valerate) may be used for the aforesaid central zone. Therefore, by using organoperoxides of differing activities at about the same temperature, the in situ formation of the built-in sealant of the lateral zones may have a greater storage modulus (G′) than the storage modulus (G′) of the central zone, and, accordingly, a reduced tendency for flow under conditions of centrifugal force occasioned by rotation of the tire.
A treatment of precipitated silica with, for example, at least one of polyalkylene glycol (e.g. polyethylene glycol) and alkoxysilane may inhibit, retard, and/or significantly prevent contact of hydroxyl groups contained on the precipitated (synthetic amorphous) silica aggregates with the organoperoxide, as well as possibly water moieties thereon. Accordingly, precipitated silica may be treated in situ within the rubber composition prior to addition of the organoperoxide, or may be pre-treated prior to its addition to the rubber composition, with a low molecular weight polyalkylene oxide polymer, which may be referred to as a polyalkylene glycol (e.g. polyethylene glycol) and/or with an alkoxysilane. It has been considered that significant challenges may be presented using the precipitated silica (optionally, also including the clay when used in combination with the precipitated silica), particularly when used in place of rubber reinforcing carbon black for reinforcing filler for a non-black colored sealant. Therefore, when the precipitated silica is used, it may be treated with one or both of a polyalkylene oxide (e.g. polyethylene oxide) and alkoxysilane.
While the butyl rubber, as a copolymer of isobutylene and isoprene, may be composed of greater than one weight percent units derived from isoprene, it may further be composed of from about 0.5 to 1.0 weight percent units derived from isoprene. The use of a butyl rubber with such low unsaturation content may promote a more efficient depolymerization by treatment with the organoperoxide where the presence of the double bonds within the butyl rubber may tend to terminate its depolymerization when the depolymerization process reaches the double bond unsaturation in the butyl rubber.
To promote better processing of the butyl rubber-based sealant precursor composition, a butyl rubber may have a relatively high Mooney viscosity (ML+8) value at 125° C. in a range from about 25 to about 60, alternately, from about 40 to about 60. Thus, a butyl rubber of very low isoprene-based unsaturation content (for more effective depolymerization of the butyl rubber) and relatively high Mooney viscosity (to promote better physical handling of the sealant precursor composition) may be used.
The butyl rubber-based sealant precursor composition may have a storage modulus (G′) physical property, at a 5 percent dynamic strain at 100° C. and 1 hertz in a range from about 170 to about 350 kPa, alternately, in a range of from about 175 to about 300 kPa. The term “phr” may designate parts by weight of an ingredient per 100 parts of elastomer, unless otherwise indicated. The terms “elastomer” and “rubber” may be interchangeable unless otherwise indicated. The terms “cure” and “vulcanize” may be interchangeable unless otherwise indicated.